Asphalt pavement milling machines have been used for the past 15 years. During this time several companies have designed and manufactured machines for both large and small milling jobs. These machines have a common denominator of high horse power and very heavy weight for cutting speed and stability. They are designed mainly for the subcontractor which specializes in asphalt milling.
One of the major problems with self-propelled milling machines used for utility trenching is they seem to sit idle much of the time because the machine can pulverize asphalt much more rapidly than the pipe laying crew can lay pipe. Expensive pieces of equipment must be utilized more continuously in order to justify their purchase investment. These specialized machines also require a trained operator and mechanic in order to utilize them efficiently.
Another problem of self propelled machines is the transportability requiring a transport truck and low boy trailer to move it to a project of any size.
The serviceability of a specialized mill may be a significant deterrent to the popularity of these machines. Milling machines have had a history of being a high maintenance item. This is partly due to being only recently developed, working in an extreme environment, being very complex with many working parts, and being subject to severe vibration continually.
There are also a number of hydraulic attachment mills for small trimming and clean up jobs. These small hydraulic driven attachments powered by the tractor's own engine are uneconomical for long or deep cuts and can't get enough trench opened when working full time for an average pipe laying contractor. Most mills of this type can not cut more than a few inches deep which can not meet the contractor's requirements for the trenching application. Only tractors with hydrostatic drive capable of moving slowly at a controlled speed with engine at maximum idle to produce adequate hydraulic power for milling can be used for these types of mills. This excludes rubber tired backhoes and loaders, the most common equipment on utility projects. Because of the above stated reasons contractors are not frequently utilizing either the small hydraulic driven machines or the large self propelled milling machines but are usually using the old method of saw cutting and hauling away asphalt chunks.
A contractor needs to be able to pulverize the trench immediately ahead of the pipelaying crew in order to reduce road closure time and traffic congestion. Thus it is necessary to have a mill sitting at the job site through out the day even though it is often used less than 10% of the time. The availability of a loader or backhoe with front bucket on pipelaying projects and utility work makes it the ideal equipment to propel this unique milling attachment. Using the tractor as the motive power for milling is a new concept in the utility trenching industry.
After calling 500 contractors involved in cutting asphalt on water lines we found that over 90% didn't have pulverizing machines. Most of those who had pulverizing machines said they were not utilizing them enough hours of the day to justify the cost.
A machine with sufficient power and speed to do the asphalt cutting for pipe laying at a cost economical to the operation has not been available. As we have experimented with different attachable milling machines we have discovered that the relationship between horse power and production is non-linear at certain points to the extent that at certain levels a slight increase in horse power will greatly increase production. The machine of this invention was designed and built to stay above this critical point at all cutting depths within its capability (12").
The milling machines built thus far are very heavy to provide for stability. Milling machines tend to vibrate and also to propel themselves rapidly out of control on the cutting mandrel if they don't have sufficient traction and weight. Using the special hookup to the tractor and using the tractor as the motive power and stabilizing support of the milling attachment allows a small unit with high horse power to mill smoothly with precession and accuracy.